In manufacturing processes of a semiconductor element, a liquid crystal display element, and so on, a circuit pattern is transferred to a resist layer via a well-known lithography process, and the circuit pattern is transferred to a specified material film (pattern forming process) by performing manufacturing processes such as an etching via this resist pattern. This pattern forming process is repeatedly performed for several times, and thereby, various circuit patterns of the material film are stacked on a substrate (semiconductor wafer and liquid crystal substrate), and circuits of the semiconductor element and liquid crystal display element are formed.
Further, in the above-stated manufacturing processes, an alignment of the substrate is performed before the lithography process, and an overlay inspection of the resist patterns on the substrate is performed both after the lithography process and before the manufacturing processes within respective pattern forming processes, to overlay various circuit patterns of the material film with high accuracy (to improve production yield). Incidentally, an alignment mark formed on a foundation layer in a last pattern forming process is used for the alignment of the substrate. An overlay mark formed on the resist layer in a current pattern forming process and an overlay mark formed on the foundation layer in the last pattern forming process are used for the overlay inspection of the resist patterns.
Besides, an apparatus detecting positions of the above-stated alignment mark and overlay mark (referred to just as a “mark” in general) is incorporated in an apparatus performing the alignment of the substrate, and an apparatus performing the overlay inspection of the resist patterns on the substrate. In the position detection apparatus, a mark being an object of detection is positioned within a visual field area, a focus adjustment is performed automatically, and thereafter, an image of the mark is scanned by an image pickup device such as a CCD camera, and a specified image processing is performed to the image of the mark, to thereby perform the position detection of the mark.
Further, an apparatus performing a focal point detection of an object plane at a time of focus adjustment is also incorporated in the above-stated position detection apparatus. The focal point detection of the object plane corresponds to a generation of a focusing signal responsive to a locational relationship of the object plane to a focal plane of an image-forming part (namely, a focused focal plane). The focusing signal is used as a control signal to match the object plane to the focused focal plane, and it is outputted to a part adjusting a relative position between the object plane and the focused focal plane (for example, a control device of a stage supporting a substrate).
As a conventional focal point detection apparatus, for example, a pupil dividing method is proposed (for example, refer to Patent document 1). In this apparatus, an object plane is illuminated via a visual field stop, images of the visual field stop are formed while dividing into two parts based on light from the object plane, and a focusing signal is generated by detecting a gap between the two images. For example, the nearer the object plane comes to an image-forming part, the larger the gap between the two images becomes, and the farther the object plane separates from the image-forming part, the smaller the gap becomes. The gap between the two images increases/decreases linearly in proportion to the locational relationship of the object plane to the focused focal plane. Besides, the gap when the object plane matches to the focused focal plane is well-known. The focusing signal is therefore generated in accordance with a difference between the gap between the two images and the well-known gap.
Patent document 1: Japanese Unexamined Patent Application Publication No. Hei 10-223517